Deadline is unclear about whether this is going to be a sequel or a prequel—and they don't know if Harrison Ford is going to be involved, either—but they say it's happening for sure:

After revisiting his classic Alien with the upcoming 3D Fox film Prometheus, Ridley Scott is committing to direct and produce a film that advances his other seminal and groundbreaking science fiction film. Scott has signed on to direct and produce a new installment of Blade Runner.